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Re: [Xen-devel] Help with reading from hvc console device on a PV guest

On Thu, Jul 23, 2015 at 10:14:57AM -0700, sainath grandhi wrote:
> Hello,
>     Please let me know if you have some information about issue below.
>     I created a channel device for PV guest using
> channel= ["connection=pty, name=xen.channel.0,
> path=/var/lib/xen/ctl-socket","connection=socket, name=xen.channel.1,
> path=/var/lib/xen/ctl-socket-data"]
>    Xen toolstack uses qemu backend for additional consoles other than console 
> 0.
> /usr/local/lib/xen/bin/qemu-system-i386 -xen-domid 505 -chardev
> socket,id=libxl-cmd,path=/var/run/xen/qmp-libxl-505,server,nowait
> -no-shutdown -mon chardev=libxl-cmd,mode=control -chardev
> socket,id=libxenstat-cmd,path=/var/run/xen/qmp-libxenstat-505,server,nowait
> -mon chardev=libxenstat-cmd,mode=control -chardev
> pty,id=libxl-channel0 -chardev
> socket,id=libxl-channel1,path=/var/lib/xen/ctl-socket-data,server,nowait
> -nodefaults -xen-attach -name ubuntupvhchannel -vnc none -display none
> -nographic -machine xenpv -m 2048
>    I see additional console devices in the guest under /dev/hvc1 and 
> /dev/hvc2.
>   I write to the other end of the channel i.e. from dom0,   but when I
> read the console device /dev/hvc1 or /dev/hvc2, using cat or an
> application using read(), I do not get the data out. But I could print
> out the data using printk (the string I passed from dom0) inside the
> hvc driver code where it reads from backend.
>   Looks like I am missing something in the userspace creation of
> device node or some flag for opening device file in the read system
> call of my application. Any suggestions?
> Application I wrote to read from console device:
>   memset(buf, 0, sizeof(buf));
>   fd=open("/dev/xenconsole/xen.channel.1", O_RDWR|O_NONBLOCK);

Have you checked if there is such device under the said directory?

You need to have udev rules in guest to create those devices for you.

See the discussion thread at


>   if(fd < 0)
>       printf("could not open dev hvc2\n");

I'm confused because the code says hvc2 here but open() has another path
in it.


>   size = read(fd, (void *)buf, sizeof(buf));
>   printf("size read %d\n",size);
> Thanks
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